A. Methanol Carbonylation to Produce Acetic Acid
For the production of acetic acid, there are three major commercialized processes, the carbonylation process, acetaldehyde oxidation process, and liquid phase oxidation process, wherein the carbonylation process accounts for about 70% of the world manufacturing capacity. Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al. on Oct. 30, 1973. The carbonylation catalyst comprises rhodium, either disolved or otherwise dispersed in a liquid reaction medium or else supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. Generally, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium, through which carbon monoxide gas is continuously bubbled. Paulik et al. disclose that water may be added to the reaction mixture to exert a beneficial effect upon the reaction rate, and water concentrations between about 14-15 wt % are typically used. This is the so-called “high water” carbonylation process.
An important aspect of the teachings of Paulik et al. is that water should also be present in the reaction mixture in order to attain a satisfactorily high reaction rate. The patentees have a large number of reaction systems as examples including a large number of applicable liquid reaction media. The general thrust of their teachings is, however, that a substantial quantity of water helps in attaining an adequately high reaction rate. The patentees also teach that reducing the water content leads to the production of ester as opposed to carboxylic acid. Considering specifically the carbonylation of methanol to acetic acid in a solvent comprising predominantly acetic acid and using the promoted catalyst taught by Paulik et al., it is taught in European Patent Application No. 0 055 618 that typically about 14-15 wt % water is present in the reaction medium of a typical acetic acid plant using this technology. It will be seen that in recovering acetic acid in anhydrous or nearly anhydrous form from such a reaction solvent, and separating the acetic acid from this appreciable quantity of water, involves a substantial expenditure of energy in distillation and/or additional processing steps such as solvent extraction, as well as enlarging some of the process equipment compared with that used in handling drier materials. Also Hjortkjaer and Jensen [Ind. Eng. Chem., Prod. Res. Dev. 16, 281-285 (1977)] have shown that increasing the water from 0 to 14 wt % water increases the reaction rate of methanol carbonylation. Above 14 wt % water, the reaction rate is unchanged.
In addition, as will be further explained hereinbelow, the catalyst tends to precipitate out of the reaction medium as employed in the process of Paulik et al., especially during the course of distillation operations to separate the product from the catalyst solution when the carbon monoxide content of the catalyst system is reduced (EP0055618). It is known that this tendency increases as the water content of the reaction medium is decreased. Thus, although it might appear obvious to try to operate the process of Paulik et al. at minimal water concentration in order to reduce the cost of handling a reaction product containing a substantial amount of water while still retaining enough water for an adequate reaction rate, the requirement for appreciable water in order to maintain catalyst activity and stability works against this end.
Other reaction systems are known in the art in which an alcohol such as methanol or an ether such as dimethyl ether or an ester such as methyl acetate can be carbonylated to an acid or ester derivative using special solvents such as aryl esters of the acid under substantially anhydrous reaction conditions. The product acid itself can be a component of the solvent system. Such a process is disclosed in U.S. Pat. No. 4,212,989 issued on Jul. 15, 1975 to Isshiki et al., with the catalytic metal being a member of the group consisting of rhodium, palladium, iridium, platinum, ruthenium, osmium, cobalt, iron, and nickel. A somewhat related patent is U.S. Pat. No. 4,336,399 issued to the same patentees, wherein a nickel-based catalyst system is employed. Considering U.S. Pat. No. 4,212,989 in particular, the relevance to the present invention is that the catalyst comprises both the catalytic metal, as exemplified by rhodium, along with what the patentees characterize as a promoter, such as the organic iodides employed by Paulik et al. as well as what the patentees characterize as an organic accelerating agent. The accelerating agents include a wide range of organic compounds of trivalent nitrogen, phosphorus, arsenic, and antimony. Sufficient accelerator is used to form a stoichiometric coordination compound with the catalytic metal. Where the solvent consists solely of acetic acid, or acetic acid mixed with the feedstock methanol, only the catalyst promoter is employed (without the accelerating agent), and complete yield data are not set forth. It is stated, however, that in this instance “large quantities” of water and hydrogen iodide were found in the product, which was contrary to the intent of the patentees.
European Published Patent Application No. 0 055 618 belonging to Monsanto Company discloses carbonylation of an alcohol using a catalyst comprising rhodium and an iodine or bromine component wherein precipitation of the catalyst during carbon monoxide-deficient conditions is alleviated by adding any of several named stabilizers. A substantial quantity of water, of the order of 14-15 wt %, was employed in the reaction medium. The stabilizers tested included simple iodide salts, but the more effective stabilizers appeared to be any of several types of specially selected organic compounds. There is no teaching that the concentrations of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid especially at low water concentrations. When an iodide salt is used as the stabilizer, the amount used is relatively small and the indication is that the primary criterion in selecting the concentration of iodide salt to be employed is the ratio of iodide to rhodium. That is, the patentees teach that it is generally preferred to have an excess of iodine over the amount of iodine, which is present as a ligand with the rhodium component of the catalyst. Generally speaking, the teaching of the patentees appears to be that iodide which is added as, for example, an iodide salt, functions simply as a precursor component of the catalyst system. Where the patentees add hydrogen iodide, they regard it as a precursor of the promoter methyl iodide. There is no clear teaching that simple iodide ions as such are of any significance or that it is desirable to have them present in substantial excess to increase the rate of the reaction. As a matter of fact, Eby and Singleton [Applied Industrial Catalysis, Vol. 1, 275-296(1983)] from Monsanto state that iodide salts of alkali metals are inactive as co-catalyst in the rhodium-catalyzed carbonylation of methanol.
Carbonylation of esters, such as methyl acetate, or ethers, such as dimethyl ether, to form a carboxylic acid anhydride such as acetic anhydride is disclosed in U.S. Pat. No. 4,115,444 to Rizkalla and in European Patent Application No. 0,008,396 by Erpenbach et al. and assigned to Hoechst. In both cases the catalyst system comprises rhodium, an iodide, and a trivalent nitrogen or phosphorus compound. Acetic acid can be a component of the reaction solvent system, but it is not the reaction product. Minor amounts of water are indicated to be acceptable to the extent that water is found in the commercially available forms of the reactants. However, essentially dry conditions are to be maintained in these reaction systems. U.S. Pat. No. 4,374,070 issued to Larkins et al. teaches the preparation of acetic anhydride in a reaction medium, which is, of course, anhydrous by carbonylating methyl acetate in the presence of rhodium, lithium, and an iodide compound. The lithium can be added as lithium iodide. Aside from the fact that the reaction is a different one from that with which the present invention is concerned, there is no teaching that it is important per se that the lithium be present in any particular form such as the iodide. There is no teaching that iodide ions as such are in significant amounts.
U.S. Pat. No. 5,001,259, U.S. Pat. No. 5,026,908 and U.S. Pat. No. 5,144,068 disclose a rhodium-catalyzed low water method for the production of acetic acid. Methanol is reacted with carbon monoxide in a liquid reaction medium containing a rhodium catalyst stabilized with an iodide salt, especially lithium iodide, along with alkyl iodide such as methyl iodide and alkyl acetate such as methyl acetate in specified proportions. This reaction system not only provides an acid product of unusually low water content (lower than 14 weight %) at unexpectedly favorable reaction rates but also, whether the water content is low or, as in the case of prior-art acetic acid technology, relatively high, it is characterized by unexpectedly high catalyst stability, i.e., it is resistant to catalyst precipitation out of the reaction medium. Employing a low water content simplifies downstream processing of the desired carboxylic acid to its glacial form.
Various means for removing iodide impurities from acetic acid are well know in the art. It was discovered by Hilton that macroreticular (macroporous) strong acid cation exchange resins with at least one percent of their active sites converted to the silver or mercury form exhibited remarkable removal efficiency for iodide contaminants in acetic acid or other organic media. The amount of silver or mercury associated with the resin may be from as low as about one percent of the active sites to as high as 100 percent. Preferably about 25 percent to about 75 percent of the active sites were converted to the silver or mercury form and most preferably about 50 percent. The subject process is disclosed in U.S. Pat. No. 4,615,806 for removing various iodides from acetic acid. In particular there is shown in the examples removal of methyl iodide, HI, I2 and hexyl iodide.
Various embodiments of the basic invention disclosed in U.S. Pat. No. 4,615,806 have subsequently appeared in the literature. There is shown in U.S. Pat. No. 5,139,981 to Kurland a method for removing iodides from liquid carboxylic acid contaminated with a halide impurity by contacting the liquid halide contaminant acid with a silver (I) exchanged macroreticular (macroporous) strong acid cation exchange resin. The halide reacts with the resin bound silver and is removed from the carboxylic acid stream. The invention in the '981 patent more particularly relates to an improved method for producing the silver exchanged macroreticular (macroporous) strong acid cation exchange resins suitable for use in iodide removal from acetic acid.
U.S. Pat. No. 5,227,524 to Jones discloses a process for removing iodides using a particular silver-exchanged macroreticular (macroporous) strong acid cation exchange resin. The resin has from about 4 to about 12 percent cross-linking, a surface area in the proton exchanged form of less than 10 m2/g after drying from the water wet state and a surface area of greater than 10 m2/g after drying from a wet state in which the water has been replaced by methanol. The resin has at least one percent of its active sites converted to the silver form and preferably from about 30 to about 70 percent of its active sites converted to the silver form.
U.S. Pat. No. 5,801,279 to Miura et al. discloses a method of operating a silver exchanged macroreticular (macroporous) strong acid cation exchange resin bed for removing iodides from a Monsanto type acetic acid stream. The operating method involves operating the bed silver-exchanged resin while elevating the temperatures in stages and contacting the acetic acid and/or acetic anhydride containing the iodide compounds with the resin. Exemplified in the patent is the removal of hexyl iodide from acetic acid at temperatures of from about 25° C. to about 45° C.
So also, other ion exchange resins have been used to remove iodide impurities from acetic acid and/or acetic anhydride. There is disclosed in U.S. Pat. No. 5,220,058 to Fish et al. the use of ion exchange resins having metal exchanged thiol functional groups for removing iodide impurities from acetic acid and/or acetic anhydride. Typically, the thiol functionality of the ion exchange resin has been exchanged with silver, palladium, or mercury.
There is further disclosed in European Publication No. 0 685 445 A1 a process for removing iodide compounds from acetic acid. The process involves contacting an iodide containing acetic acid stream with a polyvinylpyridine at elevated temperatures to remove the iodides. Typically, the acetic acid was fed to the resin bed according to the '445 publication at a temperature of about 100° C.
With ever increasing cost pressures and higher energy prices, there has been ever increasing motivation to simplify chemical manufacturing operations and particularly to reduce the number of manufacturing steps. In this regard, it is noted that in U.S. Pat. No. 5,416,237 to Aubigne et al. there is disclosed a single zone distillation process for making acetic acid. Such process modifications, while desirable in terms of energy costs, tend to place increasing demands on the purification train. In particular, fewer recycles tend to introduce (or fail to remove) a higher level of iodides into the product stream and particularly more iodides of a higher molecular weight. For example, octyl iodide, decyl iodide and dodecyl iodides may all be present in the product stream as well as hexadecyl iodide; all of which are difficult to remove by conventional techniques.
The prior art resin beds operated as described above do not efficiently and quantitatively remove impurities from acetic acid or acetic acid streams as required by certain end consumers, particularly the manufacture of vinyl acetate monomer (VAM). Accordingly, there is still a need to reduce the amounts of the impurities to a desired level in an acetic acid product stream.
B. Formation of Impurities in Methanol Carbonylation
It has been found that during the production of acetic acid by the carbonylation of methanol or methyl acetate in the presence of a finite amount of water, carbonyl impurities such as acetaldehyde, acetone, methyl ethyl ketone, butyraldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, and 2-ethyl butyraldehyde and the like, are present and may further react to form aldol condensation products and/or react with iodide catalyst promoters to form multi-carbon alkyl iodides, i.e., ethyl iodide, butyl iodide, hexyl iodide and the like. While the presence of hydrogen in the carbonylation reaction does in fact increase the carbonylation rate, the rate of formation of undesirable by-products, such as crotonaldehyde, 2-ethyl crotonaldehyde, butyl acetate, and hexyl iodide, also increases.
In rhodium-catalyzed methanol carbonylation, the formation of formic acid impurities in the product acetic acid occurs. It has been discovered that the formic acid impurity in methanol carbonylation acetic acid product is caused by the reaction of carbon monoxide and water in the reaction medium:CO+H2O→HCOOH
It has further been discovered that, under the known conditions of Rh catalyzed methanol carbonylation, the formic acid concentration in the product acetic acid is a direct function of the standing water concentration that is maintained in the carbonylation reaction medium. No other factors have been found to influence this relationship.
C. Disadvantages of Impurities
Glacial acetic acid is a raw material for several key petrochemical intermediates and products including VAM, acetate esters, cellulose acetate, acetic anhydride, monochloroacetic acid (MCA), etc., as well as a key solvent in the production of purified terephthalic acid (PTA).
Consumers of glacial acetic acid generally prefer a high purity product with as few impurities as possible and the lowest concentration on any contained impurities. The formic acid contained in product acetic acid is one such impurity and has numerous disadvantages making it an objectionable impurity for many acetic acid end uses. For example, high formic acid concentrations adversely affect the temperature and pressure control of p-xylene oxidation reactors in the terephthalic acid unit. Another example is where acetic acid is used as a feedstock for vinyl acetate (VAM) production. Formic acid impurity contained in the acetic acid generates undesirable carbon dioxide, which has to be removed from the VAM process. Traditional Monsanto technology of manufacturing acetic acid appears to produce about 175-220 ppm of formic acid in the finished acetic acid. Other methanol carbonylation acetic acid producers also produce high level of formic acid.
The iodide contamination can be of great concern to the consumers of the acetic acid as it may cause processing difficulties when the acetic acid is subjected to subsequent chemical conversion. A higher iodide environment could lead to increased corrosion problems and higher residual iodide in the final product. High iodide concentration in acetic acid could lead to catalyst poisoning problems in some downstream applications such as vinyl acetate monomer (VAM) manufacture.